Fluid vehicle dampers, such as hydraulic shock absorbers and struts, provide a smooth ride by absorbing forces that are generated by an uneven road surface. Two common types of vehicle fluid dampers are mono-tube and twin tube shock absorbers, each of which have a cylinder and piston. Twin tube shock absorbers have a valve body located at an end of the piston, commonly referred to as a piston valve, and at a cylinder end, commonly referred to as a base valve. The piston valve moves toward the base valve during compression of the shock absorber and moves away from the base valve during rebound. The valve bodies divide the shock absorbers into several fluid chambers and regulate the flow of fluid from one chamber to another thereby achieving particular ride handling characteristics. Typically, each valve body has a compression and rebound valve assembly located on opposing surfaces of the valve body that regulate fluid flow during the compression and rebound strokes. By modifying the valve assemblies, the ride handling characteristics may be calibrated.
It is desirable to have different rates of piston damping depending on vehicle ride condition. For example, during vehicle cornering maneuvers in which the piston undergoes low speed compression, it is desirable to have stiff ride handling characteristics. Conversely, when the vehicle travels over pot holes at relatively high vehicle speeds in which the piston undergoes high speed compression, it is desirable to have soft ride handling characteristics. The base valve assembly is the primary control of damping during compression. One way to best achieve different rates of damping is to provide a base valve assembly having digressive performance, that is, a base valve which provides generally independent and distinct damping rates during low and high speed piston compression. Prior art base valve assemblies have only provided low and high piston speed damping rates which are dependent on one another thereby compromising ride handling characteristics at low and high vehicle speeds. Therefore, what is needed is a base valve assembly that provides digressive performance.